1. Field of the Invention
This invention relates to an injection mold for the manufacture of golf balls and more particularly, to an injection mold for the consistent and efficient molding of golf balls having a concentric, uniform thin gage cover.
2. Prior Art
Golf balls having a core enclosed in a cover are conventionally manufactured by injection molding cover stock over the surface of a core in a split mold. The core is placed in the cavity of the split mold. The core is typically supported in place by support pins so that a space is defined between the cavity-defining mold surface and the core surface. The cover stock is injected into the space to mold the cover.
One exemplary prior art injection mold is illustrated in FIGS. 8 and 9, which are elevational and horizontal views in cross section of the mold, respectively. The injection mold generally designated at 1 includes upper and lower mold sections 1a and 1b which define a spherical cavity 2 therein by their wall surface 2a when mated at a parting plane or line P which extends coplanar with the equator of the cavity 2. It is understood that the cavity-defining wall surface 2a is provided with small projections for indenting dimples on the ball being molded, that is, a negative dimple pattern, though not shown.
As shown in FIG. 9, the mold 1 includes a main runner 6 which is disposed through the mold body in fluid communication with an injection molding machine, an annular runner 4 connected to the main runner, and a plurality of, eight in FIG. 9, molding material injecting gates 5 disposed through the mold body and radially extending from the annular runner 4 to the cavity 2 whereby molding material is injected from the molding machine to the cavity through the runners and gates. The mold 1 further includes a pair of vertically extending holes 8 disposed through the mold body at the north and south poles of the cavity and a pair of pins 9 received in the holes 8 to define gaps or vents 10 through which gases or volatiles evolving during molding escape to the exterior.
The mold 1 further includes a plurality of, four in FIG. 8 (total eight in both the upper and lower mold sections), column-like support pins 7 which are positioned on a circle about the hole 8 and circumferentially spaced apart by an angle of 90.degree., for example, for supporting a core 3 in the cavity 2. The support pins 7 each have a center axis perpendicular to the parting plane P coplanar with the cavity equator. The core 3 is held at the center of the cavity 2 by the support pins 7 in vertically opposite directions or from the upper and lower poles.
When a cover is molded over a core using the mold of the above construction, there often arise problems including eccentricity and difficulty to form a thin gage cover.
More particularly, the core is an elastic body as is the cover, the cover stock is injected under as high a pressure as 1,400 to 2,000 kg/cm.sup.2, and the gates are in a planar arrangement at the parting plane. Because of the high injection pressure and the lateral or equatorial direction in which the cover stock is injected toward the core, the core is deformed and expanded in vertical or polar directions into an elliptic or rugby ball shape. Consequently, the space between the core and the wall surface of the mold near the gates is enlarged while the space near the poles is narrowed.
At the end of injection, the cover stock partially solidifies before the core restores to its original spherical shape. Consequently, a cover of varying gage would be formed on the core. Particularly when a cover with a thin gage of 1 mm or less is formed around the core, the core can be locally exposed at the ball surface.
Similar problems occur even when a cover with a relatively thick gage is injection molded. Additionally, the load applied to the support pins by the injection pressure is considerably increased, resulting in radial misalignment.